1. Field of the Invention
This invention relates to a blade member or insert of cermet for cutting tools having a wear-resistant and thermoplastic deformation-resistant reaction layer on a surface thereof, the blade member being suited for high speed cutting.
2. Prior Art
Conventionally, cermets, containing as hard phase-constituting components a major proportion of titanium carbide (TiC) and/or titanium nitride (TiN), have been extensively used to form blade members or inserts for high speed cutting tools such as a turning tool, since such hard cermets are superior in wear-resistance to cemented tungsten carbide. However, such cermet blade members have not met the requirement of very high speed cutting operation of above 200 m/minute. Therefore, in order to meet this requirement, hard ceramics containing a major proportion of aluminum oxide (Al.sub.2 O.sub.3) have been proposed to form a blade member capable of very high speed cutting. However, such ceramics contain no binder and therefore are inferior in toughness. As a result, the use of such ceramics blade members has been limited to high speed finish cutting.
There has also been proposed and extensively used a high speed-cutting blade member made of cemented tungsten carbide and having on its surface a composite coating layer or layers composed of at least one material selected from the group consisting of TiC, TiN, titanium oxide (TiOx) and Al.sub.2 O.sub.3. Such coating layers are usually formed by a chemical vapor deposition process using reaction gas such as titanium tetrachloride, methane gas, hydrogen gas and nitrogen gas. Therefore, a deposition apparatus for forming such coating is large in size, and manufacturing costs of such surface-coated blade member are also increased. Further, the constituent parts of the deposition apparatus are susceptible to corrosion due to hydrochloric acid produced by chlorine gas generated upon decomposition of titanium tetrachloride. In addition, means for preventing leakage of such hydrochloric acid should be provided from a viewpoint of safety. Further, when the coating is formed on the blade member of cemented tungsten carbide by means of the chemical vapor deposition process, a decarburized .eta. phase is inevitably formed in its substrate immediately below the deposited coating. The presence of such a decarburized phase will lower the toughness of the blade member so that the blade member will not always have a satisfactory cutting performance.